This invention relates generally to telephony and more particularly to a low power dissipation, series regulator for use with a pulse code modulated (PCM) transmission line to limit current used to power repeater amplifiers along the transmission line.
In a PCM transmission system a number of analog signals are digitized, multiplexed and transmitted as bipolar digital pulses on a transmission line. The transmission line is a two conductor cable. In order to maintain timing and amplitude of the PCM bipolar pulses, repeater amplifiers are interposed at predetermined intervals to amplify and reshape the bipolar pulses. In order to provide power to the repeater amplifiers along the transmission line the two conductors carry positive or negative d.c. current in addition to carrying the bipolar pulses.
In existing PCM systems the negative (or positive) conductor is provided with a current regulator to assure proper operation of the repeater amplifiers during normal operation. The other conductor (positive or negative) is unregulated and only has a fuse to protect it. In such a conventional system, a short circuit from the positive conductor to ground either accidentally or due to the extinguish time of a current surge protector (generally a gas discharge tube) may, after a short time (less than two seconds), cause a terminal carrier group alarm (CGA) which will disconnect all the PCM users for 20 seconds. Furthermore, the short circuit current or the protector's conduction currrent may blow the fuse in the positive conductor thereby making the entire system inoperative and requiring maintenance of the system before it can be used again to carry information.
One solution to the problem of short circuits in the positive conductor of a PCM transmission line is to provide a current regulator which would limit the current to allow the surge protector to extinguish or protect the equipment until the short is removed. However, a conventional current limiting circuit is not practical due to the high power dissipation with its accompanying heat that would result if the short circuit existed over an extended period of time. Therefore, it is necessary to provide a series regulator in the positive conductor which, in case of an extended short circuit condition, will cycle on and off until the short circuit condition ceases at which time the regulator will automatically resume normal operation. In order to minimize the power dissipated by the series regulator during the short circuit condition, the regulator must operate at a low duty cycle. In other words the regulator must be on for less time than it is off. Moreover, the duty cycle timing also must not be significantly dependent upon either the ambient temperature or the short circuit line current.
The prior art does not provide a series regulator which can adequately solve the need for a low power dissipation series regulator in a PCM transmission line. For example, U.S. Pat. No. 3,903,475 issued to Mokrani et al. shows a current limiting circuit having a pair of series pass transistors which are controlled by a time out circuit which shuts the system down after an overload condition persists for a predetermined period such as two or three seconds. There is no provision made in the Mokrani patent to automatically turn on the series regulator again in order to resume operation after the overload condition has ceased.
U.S. Pat. No. 3,959,713 issued to Davis et al. discloses a series pass regulator which provides a temperature sensing device which controls a series pass transistor. The temperature sensing device turns off the series pass transistor in response to an increase in temperature in the sensing device resulting from an overload condition. After the series pass transistor has been turned off and the heat in the sensing device has dissipated, the temperature sensing device turns on the series pass transistor again so as to restart the series regulator. The process is repeated until the overload condition has ceased, and the regulator of the Davis et al. patent then operates in its normal mode. However, the operation of the Davis et al. regulator is dependent both on the ambient temperature and the short circuit current. As a result, the maximum current required to induce shut down varies as well as the ratio of the on and off times for the series regulator during the overload condition.
It is an object of the present invention to provide a two terminal series regulator for a d.c. current conductor of PCM transmission line which, in the presence of an overload condition, will operate for a first predetermined time at a predetermined, predictable maximum overload current level, which will then provide a low power shut down condition for a second predetermined time, and which will alternatively turn itself on and off with a predictable, low duty cycle until the overload condition ceases.
It is further an object of the present invention to provide a series regulator for the d.c. current conductor of a PCM transmission line which has a duty cycle and a maximum current level that are not significantly dependent on the ambient temperature or the overload line current.
It is also an object of the invention to provide a timer for the series regulator for timing the first and second predetermined times, thereby establishing the series regulator's duty cycle, which timer is energized solely by the voltage drop across the terminals of the series regulator.
Collaterally it is an object of the invention to provide a timer having a reference voltage diode and a current source to assure that if the voltage drop across the series regulator exceeds a threshold voltage, the voltage supplied to a timing circuit in the timer will be substantially constant and thereby assure predictable timing of the series regulator's duty cycle .